The overlapping flaps which form the end panel of a beverage can carton are commonly held in place by a locking arrangement between coacting tabs on the outer end panel flap and apertures in the inner end panel flap. Each locking tab has a heel portion, which is held in place adjacent a locking edge of its associated aperture, and a toe portion, which is wider than the aperture into which it is inserted.
One problem with this type of lock stems from the fact that the toe portion of the locking tab must be substantially wider than the opening of the locking aperture in order to prevent the tab from being pulled out during handling or shipping of the carton. It also has to be wide enough to allow for slight misalignment of the tab and the aperture during carton fabrication in the packaging machine. Countering this requirement, however, is the need to be able to readily insert the tabs into the apertures so as avoid carton failures due to inadequately inserted tabs. This has resulted in attempts to ease the insertion of the tabs without decreasing the width of the toe portion to the point where the holding power of the locking mechanism is impaired.
One such attempt is disclosed in U.S. Pat. No. 3,955,748 issued to Calvert. According to the design of this patent the bottom edge of the aperture is provided with a centrally located notch and the tab is provided with a centrally located fold line running the length of the tab. When the tab is first inserted its movement into the aperture is impeded by its contact with the paperboard immediately surrounding the aperture, causing the tab to fold along the longitudinal fold line and to enter the aperture through the clearance notch. One problem with this arrangement is that the tab has to fold quite sharply in order to enter the clearance notch, but the design of the tab, wherein the fold line extends into a region of the tab which is connected to the panel flap itself, prevents the tab from having this capability. Another problem with the arrangement is that if the tab is misaligned to the extent that only one side of the toe portion enters the aperture, an event made more likely by the requirement that the toe portion enter the aperture through the clearance notch, the contour of the bottom edge of the aperture is not enough to assure that the tab will be held in place.
Another locking arrangement intended to improve upon the design described above is disclosed in U.S. Pat. No. 4,577,799 issued to Oliff. In that arrangement a holding tab is provided at the bottom edge of the aperture to prevent the locking tab from being forced out of the aperture. To facilitate the entry of the toe portion of the tab into the aperture the tab is provided with a longitudinal fold line as in the Calvert design and the tab edge is made arcuate in shape. The tab does not have to fold as much as it does in the Calvert arrangement because the toe portion has additional space in which to enter the aperture at the edges of the aperture adjacent the ends of the arcuate holding tab, as shown in FIG. 5 of the drawing. The arcuate shape of the holding tab, however, restricts contact between the locking tab and the holding tab to a relatively small centrally located area of the locking tab. Further, because of the limited ability of the locking tab to fold there is no assurance that the tab will be inserted sufficiently in the event of misalignment of the tab and the aperture.
While it would obviously be desirable to provide a holding tab that affords greater locking tab contact, it does not appear to be feasible in view of the inability of the toe portion of the locking tab to fold to the extent necessary to move past such a holding tab.